Use Constructions to Show Slope Criteria for Parallel and Perpendicular Lines

Print Lesson


Students will be able to show the slope criteria for parallel and perpendicular lines by construction.

Big Idea

By folding origami, students will review angle relationships and build a foundation for later work.

Launch Origamics!

1 minutes

Warm-Up: Origamics Problem

15 minutes

In the Origamics Problem, students follow directions to fold their square paper to unearth important angle relationships.  This problem provides students with a hands-on way to physically show angle relationships, as well as multiple ways to justify one's reasoning for how to find the measure of all angles.  


Show Slope Criteria for Parallel and Perpendicular Lines

10 minutes

To review constructing perpendicular and parallel lines in a coordinate geometry context, I have my students:

  1. Graph a line segment
  2. Perpendicularly bisect it
  3. Construct a perpendicular through a point to the line segment

Through this exercise my students use geometry ideas to deepen their conceptual understanding of why lines that are parallel have equal slopes and why lines that are perpendicular have slopes that are negative reciprocals of each other.

Afterward, a whole-class discussion provides an opportunity for me to preview how we will prove the slope criteria for parallel and perpendicular lines, which we will do later on the year when we get to the unit on Triangle Similarity.



Constructions Problems

20 minutes

This in-class asks students to use their constructions skills to successfully complete four constructions problems. As I launch this task, I ask my students to first sketch and label the figure they have been asked to construct -- this is important because students sometimes begin their constructions without an idea of what their construction should ultimately look like. Making a sketch sparks gives them a perspective on the problem from which to imagine a plan of attack. I tell my students to first create an image and a plan, then construct the figure using a compass and straightedge. Afterward, describe the steps of your construction in a few short sentences.



Homework: Constructions Review

5 minutes

I give this Constructions Review Homework because it asks students to review foundational constructions, which are essential as we near the end of this unit.  I ask students to reflect on their level of understanding when we correct this homework in class, which often gives students an idea of their level of success with constructions.